The present invention relates to a coating apparatus and a coating method for coating a flexible support with coating compositions at a high speed, and particularly to a coating apparatus and a coating method in which a continuously running flexible support is laminate-coated with coating compositions such as magnetic coating compositions multiply and simultaneously.
Heretofore, magnetic recording media such as a magnetic tape have been used generally as information recording media for information apparatuses such as video apparatuses, audio apparatuses, computers, etc. There are various methods and apparatuses for producing this type magnetic recording media. As one of the producing methods, a method of applying a coating composition such as a magnetic dispersion onto a film-like flexible support (hereinafter referred to as "support") formed of polyethylene terephthalate to thereby form a recording layer on the support is used widely because high efficiency in production is attained by this method. As a method of coating a coating surface of the support with a coating composition, there is a method using an extrusion type coating apparatus. As the method using an extrusion type coating apparatus, there is well-known a method in which a coating head having a slot for discharging a coating composition is urged against a surface of the support so that the discharged coating composition is applied to the surface of the support uniformly at a doctor edge portion. The shape of the doctor edge, the pressure of the coating composition, the distance between the support and the position of the top end of the doctor edge, and so on, have been disclosed, for example, in Unexamined Japanese Patent Publication (OPI) Nos. Sho. 60-238179, Sho. 62-117666, Sho. 62-132566 and Hei. 4-145977 and have been discussed variously.
When this coating method using an extrusion type coating apparatus is used, a multiplicity of coating layers can be formed on a surface of a support by discharging a plurality of coating compositions from a plurality of slots. By providing functions separated by the layers, a product having such excellent characteristic that a conventional coating support lacks can be produced. The method of for example, in Unexamined Japanese Patent Publication (OPI) producing such a multi-layered product has been disclosed, No. Sho. 63-88080 Japanese Patent Application No. Hei. 4-47746, and so on.
In the coating performed by the aforementioned extrusion type coating apparatus, however, a good coating layer cannot be formed until the balance between conditions such as the urging force of the edge surface of the coating head against the surface of the support, the discharge force of the coating composition, etc. is maintained. Particularly in the case of a coating method for forming two or more coating layers multiply and simultaneously, coating compositions ejected from respective slots are mixed with each other to form a mixed coating layer if positions of respective edge surfaces relative to the support are slightly different. Accordingly, the characteristic of the resulting coating layer becomes complex, so that it is difficult to produce a product having a multiplicity of coating layers formed thereon.
It is therefore necessary that factors of conditions such as the tension of the support, the position of the upstream pass roller on the upstream side in the direction of running of the support, the position of the downstream pass roller on the downstream side in the direction of running of the support, the shape of each edge surface, the position of each edge surface relative to the support, etc. are adjusted on the basis of experience to determine optimum conditions. There is however the case where good coating layers cannot be formed in accordance with the changes of the material quality, thickness, etc. of the support though optimum coating conditions are selected. It is therefore necessary that optimum conditions are retrieved and that a coating process is determined by testing on the basis of the retrieved optimum conditions whenever the material quality, thickness, etc. of the support are changed. It is the present situation that a large amount of time is required for such retrieval. Particularly in high-speed thin layer coating, the time required for retrieval of coating conditions becomes a serious issue because the adjustment of coating conditions is very delicate and extremely difficult.